This invention relates to an exposure slit apparatus for use in an optical system of a reproducing machine preferably of the electrostatographic type. The machine includes means for copying documents selectively at one of a plurality of magnifications.
In an optical system for stripwise viewing a document the projected illumination intensity tends to fall off at the edges of the image as compared to the center thereof. To compensate for this problem exposure slits have been employed which allow a greater portion of the projected image rays to pass through the slit at the edges of the image than in the middle thereof. It has been common practice to employ such a slit either directly above the imaging surface or at a position nearer the object plane.
In at least one machine the Xerox 840 machine two exposure slits are utilized, one near the object plane and the other near the image plane or xerographic drum. In machines such as the Xerox 840 which provide a plurality of projected image magnifications further complications occur since it is necessary to compensate not only for the normal fall off in illumination toward the edges of the image, but also for the changes in illumination resulting from the magnification changes. For example, the irradiance at the drum increases with increasing reduction of the projected image. The available exposure time decreases with increasing image reduction. The fall off in intensity at the edges of the image is less with increasing image reduction since the field angle decreases.
It should be apparent from the foregoing that an illumination slit in the 1X mode of operation would probably not provide uniform illumination for a reduction mode of operation. Similarly, a slit configured with an appropriate profile for uniform illumination in a reduction mode of operation would not provide uniform illumination for a 1X mode of operation or for a further reduced mode of operation. This is the case since the proper slit profile is affected by the aforementioned factors for each of the modes of operation.
Various means have been devised for compensating for these problems. It is possible to use "active" slits whose width is adjustable to provide the appropriate slit profile for each mode of optical exposure. In accordance with the teachings of U.S. Pat. No. 3,917,393 to Nier an inclined "passive" exposure slit system is provided which is capable of achieving a change in the effective slit profile for compensating for magnification change as well as illumination fall-off.
Still another approach is found in the Xerox 840 machine as described above which employs two passive illumination slits. The illumination slit near the image plane or xerographic drum has a profile which is appropriate for a 1X or base mode of operation. The illumination slit near the object plane has a profile whose projected image at the image plane in a reduction mode of operation is effective to properly control illumination during reduction copying. In the 840 machine the center line of the object plane slit projected at the image plane and the center line of the image plane slit are coincident so that the projected object plane slit falls inside the boundaries of the image plane slit in a reduction mode and outside the boundaries of the image plane slit in a 1-X mode or base mode. This is achievable in that machine because the optical system is configured so that the center lines of the projected images are coincident regardless of optical mode.
It has been found that in a compact xerographic machine wherein multiple modes of varying optical projected image magnifications are provided that design constraints are imposed by the space available in the optical cavity and the desire to avoid vignetting and other associated problems. Therefore, it is desirable in some cases to move the center line of the projected image in one magnification mode relative to the center line of the projected image in an alternative magnification mode. Therefore, it is proposed in accordance with this invention to modify the two illumination slit approach presently utilized in the Xerox 840 machine to provide an illumination slit system which is adapted for controlling illumination in a machine wherein the projected image center lines are displaced from one another depending on the optical modes. The system of the Xerox 840 machine would not be appropriate in such a case because the projected image in reduction of the object plane slit would not fall completely within the boundaries of the image plane slit and, therefore, part of the illumination would be lost and uniform illumination profile would not be achieved.
A variety of electrostatographic reproducing machines are commercially employed which have different modes of operation. One type of machine utilizes a moving original exposure system wherein an original document is moved past a fixed slit optical system for projecting an image onto the moving photoconductive surface. These machines include a means for changing the magnification of the projected image to provide reduction copies. Exemplary of patents in this area is U.S. Pat. Nos. 3,076,392, to Cerasani et al., and 3,649,114 to Vlach et al.
Other machines have been adapted to copy stationary original documents at a variety of magnifications or reductions through the use of a scanning optical system. Exemplary of patents in this area are U.S. Pat. Nos. 3,476,478, to Rees, Jr.; 3,542,467 to Furgeson; 3,614,222 to Post; and 3,837,743 to Amemiya. Another approach which has been utilized for projecting images for reproduction at varying magnifications from a stationary original comprises full frame exposure. Exemplary of patents in this area are U.S. Pat. Nos. 3,543,289 to Koizumi; 3,687,544 to Muller; 3,703,334 to Knechtel; and German Offenlegungsschrift No. 2,154,944 to Libby.
U.S. Pat. Nos. 3,703,334 to Knechtel and 3,837,743 to Amemiya set forth above are also significant in that they disclose the use of a separate reflector or add reflectors, respectively, which are selectively positionable in the optical path for changing the conjugate distance of the optical system for providing varying magnifications.
The aforenoted machines are adapted to provide one or more modes of copying having different magnifications. In the optical systems of these machines, some means is usually provided for changing the conjugate relationship of the object and image sides of the projection lens. This may be accomplished by translating the lens between different positions for different projected image magnifications or in accordance with an alternative approach by utilizing more than one lens whereby different lenses are selectively positionable in the optical path, depending upon the projected image magnification desired. See, for example, U.S. Pat. No. 3,779,642 to Ogawa.
Other forms of multi-mode copiers are available commercially. For example, in the Xerox 3100 LDC machine an optical system is provided which enables the machine to copy from a stationary original in a first scanning mode or from a moving original in a second fixed optical mode. This latter mode is particularly adapted for copying documents larger than the conventional viewing platen size. U.S. Pat. No. 3,900,258 to Hoppner et al. [1] is illustrative of a machine similar in many respects to the 3100 LDC machine.
Reproducing apparatuses including the capability of making copies from both moving and stationary originals are also described in U.S. Pat. No. 3,833,296 to Vola, and in IBM Technical Disclosure Bulletin, Vol. 12, No. 1, at page 173, June 1969.
It has been found desirable, to provide a multi-mode reproducing apparatus having various unique features of the 3100 LDC machine, including its extremely compact size, but also having the capability of reduction copying.
One approach to such a machine is described in U.S. application Ser. No. 588,971, filed June 20, 1975, to Hoppner et al. [2]. In that application a multi-mode reproducing apparatus is provided including both moving and stationary original exposure modes, with at least two modes of moving original exposure at differing copy image magnifications. The Hoppner et al. [2] machine includes a first optical mode wherein a stationary document is viewed and an image thereof is projected onto a moving imaging surface. In a second mode a document moving at a first speed synchronized to the speed of the moving imaging surface is viewed and an image thereof projected onto the imaging surface at a desired magnification. In a third mode a document moving at a second speed synchronized to the speed of the moving imaging surface is viewed and an image thereof projected onto the surface at a reduced magnification. In the reduction mode of operation a scanning optical system is held fixed at a given position and an additional optical element comprising an add mirror in inserted into the optical path in order to change the conjugage on the object side of the lens. The lens is also shifted to a new position to align it with a repositioned optical path.